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Just FYI: what you are calling the "plug" is the magazine.
Clean it really well, meaning take everything apart, wipe down, oil, wipe oil off. Put it back together. If still wont work have a smith look at it, if he cant fix it buy a benelli
Remington shotguns have a spring steel cartridge stop at each side of the receiver. One retracts to let a shell come out of the magazine tube into the loading mechanism, and one stops the next shell from coming out until the action is cycled again. Over time they get worn or bent. Best fix is to have a gunsmith replace them. The new parts should be staked into the receiver grooves with a tool that is made specifically for the job. Chances are that the parts got bent out of shape when the gun was disassembled for cleaning, and then reassembled without pushing the parts out of the way to get the bolt assembly back into the reciever.